In automation technology, actuators are often controlled by digital outputs of a control device, for example a programmable logic controller (PLC). Apart from controlling automated processes, safety-related functions are also increasingly controlled by control devices and carried out by corresponding actuators. To avert damage to people, machines and the environment, increased requirements apply to such safety-related functions with regard to their functionality, i.e. in particular with regard to the reliability and availability of the executing controllers and the activated actuators.
The digital signal outputs of a safety-related control device may have a monitoring device, with which a respective signal output current circuit can be monitored for faults in the wiring and for disturbances in the input region of a connected actuator.
In particular when operating actuators that have to be supplied with electrical energy, the signal output current circuit of the digital signal outputs of a control device is often electrically isolated from the respective load current circuit of the actuator as the digital signal outputs can in general drive only limited currents. The decoupling of the signal output current circuit and the load current circuits takes place, for example, by means of coupling relays, which, depending on an input signal provided by the respective digital signal outputs, can switch at least one load current circuit. The switching elements or the relay contacts in the load current circuit of the coupling relay can be configured both as a break-contact element and in the form of a make-contact element, so an inverting or non-inverting switching function can be realised with the coupling relay.
The corresponding coupling relays that meet the requirements for operation on a safety-related control device are known as safety relays. Depending on the safety requirement, a safety relay suitable for safely switching on an actuator comprises up to three switching elements connected in parallel. The switching elements are redundantly activated by a control logic of the safety relay depending on an input signal provided by the digital signal outputs of the control device so the load current circuit can be redundantly switched on by means of the switching elements.
A substantial drawback in the decoupling between the signal output current circuit and the load current circuit by a safety relay is that the monitoring device of a safety-related control device can only monitor the input current circuit of the safety relay. Possible faults in the load current circuit, for example wiring faults and faults within the safety relay, for example in the form of a failure of the internal control logic or individual switching elements, however, cannot be detected by the monitoring device of the digital signal outputs of a safety-related control device.